Sourcemaps support for RAM

Summary:This rev adds support for production sourcemaps on RAM.

When we inject a module into JSC we use the original `sourceURL` and specify the `startingLineNumber` of the module relative to a "regular" bundle. By doing so, when an error is thrown, JSC will include the provided `sourceURL` as the filename and will use the indicated `startingLineNumber` to figure out on which line the error actually occurred.

To make things a bit simpler and avoid having to deal with columns, we tweak the generated bundle so that each module starts on a new line. Since we cannot assure that each module's code will be on a single line as the minifier might break it on multiple (UglifyJS does so due to a bug on old versions of Chrome), we include on the index the line number that should be used when invoking `JSEvaluateScript`. Since the module length was not being used we replaced the placeholder we have there for the line number.

Reviewed By: javache

Differential Revision: D2997520

fb-gh-sync-id: 3243a489cbb5b48a963f4ccdd98ba63b30f53f3f
shipit-source-id: 3243a489cbb5b48a963f4ccdd98ba63b30f53f3f

local-cli/bundle/output/unbundle/write-sourcemap.js

@@ -16,7 +16,7 @@ function writeSourcemap(fileName, contents, log) {
     return Promise.resolve();
   }
   log('Writing sourcemap output to:', fileName);
-  const writeMap = writeFile(fileName, '', null);
+  const writeMap = writeFile(fileName, contents, null);
   writeMap.then(() => log('Done writing sourcemap output'));
   return writeMap;
 }